The present invention relates to an electromagnetic stepping motor for two directions of rotation, consisting essentially of
a) a stator (1) having four half-coils (2) with iron cores (4), the half-coils being arranged symmetrically with respect to a rotor shaft (3) wherein every two opposite ones of the half-coils are electrically connected to each other and form a coil controlled to produce magnetic fields in the same direction, the stator having a return bridge (5) which magnetically connects the iron cores (4) to each other; PA1 b) a rotor (6) which is rotatably mounted in the stator (1) and has a two-pole or six-pole permanent magnet assembly (7); as well as PA1 c) a housing (23) enclosing the stator and the rotor. PA1 (a) iron cores (4) arranged equidistant from and parallel to the rotor axis (3) with axes perpendicular to a return bridge (5); PA1 (b) wherein each half-coil (2) is wound in the direction opposite to the half-coil (2) opposite it; PA1 (c) wherein the iron cores (4) are connected magnetically to each other by a pole-shoe spider (8) located at their ends lying opposite the return bridge (5); and PA1 (d) wherein a permanent magnet (7) of the rotor (6) is arranged, with the formation of an air gap, within a central bore hole (9) of the pole-shoe spider (8).
Stepping motors for two directions of rotation are frequently developed with an iron-free cross-coil winding as shown in Federal Republic of Germany OS 38 28 464, in which the magnet, which is connected, fixed for rotation with the rotor, lies completely within the coil. These stepping motors have the advantage that a purely sinusoidal torque can be produced with them, as is important for action in a microstep process.
There is the disadvantage, however, that the nature of the winding requires a certain minimum structural height, so that an extremely flat construction cannot be realized, or can be realized only with considerable limitations.
A stepping motor which is of a basically different manner of construction is disclosed in German Patent 30 26 004. In that case, two coils are wound on a stator having pole shoes the axes of which are arranged in the plane of the stator at an angle of about 120.degree. to each other. A total of three pole shoes are provided which are separated from each other by air gaps and surround the two-pole magnet of the rotor. Such stepping motors enable a substantially flatter construction but they have a very high pole sensitivity, i.e. only providing a torque which insufficiently approximates sinusoidal shape, wherein differences from sinusoidal shape must be individually taken into account for control in the microstep process.
Stepping motors are also known (Federal Republic of Germany OS 39 18 538) in which a stator 4 has inward facing pole shoes on which four half-coils are arranged. In this case, there must be a sufficiently large free space in the center of the stator in order that the coils can be pushed or wound on the pole shoes. This necessarily means a rotor of relatively large diameter and correspondingly low start-stop frequency. Furthermore, there is extensive pole sensitivity due to the gaps between the pole shoes.